Expose per-role resource allocation metrics.

We currently expose some global resource allocation metrics across all roles, e.g.:

    "master/cpus_percent": 0.6737,
    "master/cpus_revocable_percent": 0,
    "master/cpus_revocable_total": 0,
    "master/cpus_revocable_used": 0,
    "master/cpus_total": 100,
    "master/cpus_used": 67.37,

It would be good to expose metrics for tracking this on a per-role basis, so that this can be easily graphed without having to also collect information from the /roles endpoint.